The present invention relates to a method for producing poly(p-t-butoxystyrene). In more detail, the present invention relates to a method for producing poly(p-t-butoxystyrene) characterized in that a mixed solvent composed of a specific ether compound, a diether of (poly)alkylene glycol and a hydrocarbon solvent is used as a polymerization solvent.
Poly(p-t-butoxystyrene) has been well known as a variety of functional resins such as a resist material used for producing VLSIs. As an example of methods for its production, a method in which p-t-butoxystyrene is polymerized in a variety of solvents using an organometallic compound as a polymerization initiator has been known.
For instance, a method in which an organometallic compound, such as an alkyllithium and sodium naphthalene, is used as a polymerization initiator and a single solvent, such as an aromatic hydrocarbon, a cyclic ether and an aliphatic hydrocarbon, is used has been proposed in JP-B63-36602 and JP-A-6-123970.
This method, however, has an industrial drawback that it requires a polymerization facility in which polymerization is performed at ultra low temperatures of from xe2x88x9270 to xe2x88x9278xc2x0 C., although it can afford a polymer with a relatively narrow molecular weight distribution.
In order to overcome the aforementioned drawback, for example, a method in which a mixed solvent composed of a hydrocarbon solvent and a polar solvent such as tetrahydrofuran, 1,4-dioxane, ethyl ether and N-methylpyrrolidine and sec-butyllithium as a polymerization initiator has been proposed in JP-A-6-298869.
This method, however, is not necessarily satisfactory in a rate of forming the target compound, etc. Therefore, there has been a demand for improving the method in this respect.
Under these circumstances, the present inventors have extensively studied on reaction solvents in order to improve the rate of forming the target compound, etc. As a result, they have found that the use of a mixed solvent composed of a specific ether compound, namely, a diether of (poly)alkylene glycol and a hydrocarbon solvent can remarkably improve the rate of forming the target compound and the like, and can provide the target compound efficiently. They have further conducted the study and accomplished the present invention.
The present invention provides an industrially advantageous method for producing poly(p-t-butoxystyrene) by polymerizing p-t-butoxystyrene in which an organometallic compound is used as a polymerization initiator and a mixed solvent comprising a hydrocarbon solvent and a diether of (poly)alkylene glycol is used as the polymerization solvent.
The present invention is characterized in that a mixed solvent composed of a hydrocarbon solvent and a diether of (poly)alkylene glycol is used as the polymerization solvent. Examples of the diether of (poly)alkylene glycol include compounds represented by the following formula:
R1(OR2)nOR3
wherein R1 and R3 represent a lower alkyl group, R2 represents a lower alkylene group and n represents a number of from 1 to 5.
Examples of the lower alkyl group as R1 or R3 include methyl, ethyl, propyl, i-propyl, butyl, i-butyl and sec-butyl. Examples of the lower alkylene group as R2 include ethylene, propylene, butylene and pentylene.
Representative compounds represented by the foregoing formula include diethers of alkylene glycols such as ethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol dipropyl ether, ethylene glycol dibutyl ether, ethylene glycol methyl ethyl ether, ethylene glycol methyl butyl ether, propylene glycol dimethyl ether, propylene glycol diethyl ether, propylene glycol dipropyl ether and propylene glycol dibutyl ether; and diethers of polyalkylene glycols such as diethylene glycol dimethyl ether, triethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether.
The above exemplified compound can be used singly or in combination of two or more of them, as the diether of (poly)alkylene glycol.
Representative examples of the hydrocarbon solvent include aromatic hydrocarbons such as benzene, toluene and xylene, and aliphatic hydrocarbons such as hexane, heptane, cyclohexane and methylcyclohexane.
The above exemplified compound can be used singly or in combination of two or more of them, as the hydrocarbon solvent.
Examples of the preferred mixed solvent include hexane/ethylene glycol dimethyl ether, hexane/ethylene glycol diethyl ether, hexane/ethylene glycol dibutyl ether, and toluene/ethylene glycol dimethyl ether.
In the present invention, the diether of (poly)alkylene glycol is used preferably in the amount of from 0.05 to 10% by weight, more preferably in the amount of from 0.2 to 5% by weight, based on the hydrocarbon solvent. When the amount exceeds 10% by weight, side reactions such as a coupling reaction tend to occur. On the other hand, when the amount is less than 0.05% by weight, there is a tendency to cause reduction in yield due to decrease in reaction rate and side reactions such as chain transfer. In both cases, there is a tendency to get worse in molecular weight distribution, dispersion and the like.
The mixed solvent is usually used in the amount of from 5 to 20 times by weight based on p-t-butoxystyrene.
The organometallic compound used in the present invention as a polymerization initiator include, for example, organoalkaline metals. Examples of organoalkaline metals include organolithium compounds, such as n-butyllithium, sec-butyllithium, i-butyllithium, t-butyllithium, 2-methylbutyllithium and lithium naphthalene; and organosodium compounds, such as sodium naphthalene, sodium anthracene, sodium xcex1-methylstyrene tetramer and sodium biphenyl. Among them, n-butyllithium and sec-butyllithium are preferably used.
Although the amount of the organometallic compound varies depending upon a molecular weight of the desired poly(p-t-butoxystyrene), usually about 10xe2x88x925 to 10xe2x88x923 mole per gram of the monomer, p-t-butoxystyrene is preferred.
The polymerization reaction is usually carried out in a high vacuum or under an inert gas atmosphere such as argon and nitrogen. The reaction temperature is usually from xe2x88x9250 to 0xc2x0 C., preferably from xe2x88x9240 to 0xc2x0 C., more preferably from xe2x88x9240 to xe2x88x9220xc2x0 C. The reaction time usually ranges from 10 minutes to 20 hours.
The polymerization reaction can be terminated by addition of a small amount of a polymerization terminator such as water and methanol.
Subsequently, the reaction mixture is subjected to washing with water and removal of the organic solvent by distillation to isolate the desired poly(p-t-butoxystyrene). The resulting poly(p-t-butoxystyrene) can be purified by adding it into methanol or the like after the distillation of the organic solvent.
The molecular weight distribution can be calculated by determining a number average molecular weight (Mn) and a weight average molecular weight (Mw) in terms of polystyrene by GPC analysis. The peak pattern of the resulting GPC elution curve shows the degree of monodispersion.
According to the present invention, the desired poly(p-t-butoxystyrene) can be produced efficiently and readily under relatively mild conditions using a mixed solvent composed of a specific ether compound, namely, a diether of (poly)alkylene glycol and a hydrocarbon solvent as a polymerization solvent.